This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2010-154872 filed in Japan on Jul. 7, 2010, the entire contents of which are herein incorporated by reference.
The present invention relates to an electrophotographic color image forming apparatus such as a color printer, a color copying machine, or a color facsimile machine.
In color image forming apparatuses of this type, there is the problem of color shifts since a color image is formed by superimposing images of a plurality of colors. For example, a color image forming apparatus has a configuration in which latent images are respectively written on a plurality of latent image carriers (photosensitive drums) corresponding to a plurality of colors, the latent images on the latent image carriers are developed, and images of the colors are respectively formed on the latent image carriers, transferred from the latent image carriers to an image carrier (transfer belt), superimposed and formed on the image carrier, and further transferred from the image carrier to a recording medium, whereby a color image is formed on the recording medium. In such a color image forming apparatus, images of a plurality of colors may be shifted when the images are transferred from the latent image carriers to the image carrier, and thus a color shift may occur, which deteriorates the quality of a color image.
For this reason, in the invention described in JP 2009-150997A, color shift detection patterns of a plurality of colors are formed in an image formation area of an image carrier (transport belt), shifts of the color shift detection patterns of the colors are detected, and the positions of images of the colors to be formed in the image formation area of the image carrier are corrected based on the shifts of the color shift detection patterns of the colors, thereby preventing color shifts.
However, in the case where such color shift detection patterns of a plurality of colors are formed in the image formation area of the image carrier and detected as in the invention described in JP 2009-150997A, when a normal image is formed in the image formation area, the color shift detection patterns of the colors cannot be formed in the image formation area. Accordingly, it has been necessary to form and detect the color shift detection patterns at the interval of image formation operations or in a state in which an image formation operation is stopped, and thus it has been necessary to set a special period for this.
In the invention described in JP 2005-99716A, an image formation area in the center in the width direction of an image carrier (transfer belt) and image non-formation areas at both edges in the width direction thereof are distinguished, and thus it is possible to detect pitch detection patterns in the image non-formation areas even when an image is formed in the image formation area of the image carrier.
However, the degree of, for instance, color shifts due to so-called bow (curved scanning lines from a light scanning apparatus to a photosensitive body) and the like differs in the center of the image carrier and at both edges thereof. Accordingly, as in the invention described in JP 2005-99716A, even if detection patterns are provided in the image non-formation areas at both edges in the width direction of the image carrier and detected, and then the detection result is used for correction of an image in the center of the image carrier, the correction error was large.
On the other hand, in the invention described in JP 2007-65500A, although bow, for instance, is reduced by pressing a long lens through which scanning beams for latent image writing pass, it is difficult to effectively eliminate color shifts due to bow and the like with such a method. Accordingly, even if the invention described in JP 2005-99716A is combined with the invention described in JP 2007-65500A, color shifts in the center of the image carrier could not be accurately corrected.
As described, in the invention described in JP 2009-150997A, color shift detection patterns of a plurality of colors are formed in the image formation area of the image carrier and detected, and thus it has been necessary to form and detect the color shift detection patterns at the interval of image formation operations or in a state in which an image formation operation is stopped, and thus it has been necessary to set a special period for this.
In the invention described in JP 2005-99716A, since detection patterns in the image non-formation areas of the image carrier are detected, even when an image is formed in the image formation area of the image carrier, the detection patterns can be detected. However, even if the detection result is used for correction of an image in the center of the image carrier, a correction error was large due to the influence of bow and the like.
Furthermore, in the invention described in JP 2007-65500A, although bow, for instance, is reduced by pressing the long lens through which scanning beams pass, it is difficult to effectively eliminate color shifts due to bow and the like. Accordingly, even if the invention described in JP 2005-99716A and the invention described in JP 2007-65500A are combined, color shifts in the center of the image carrier could not be accurately corrected.